I have a Mac Pro that draws about 250 watts (so I read somewhere) when it's running full-bore. It apparently draws less than 90 when it's sleeping. For whatever reasons I like to leave it on overnight, but I'd like it to sleep for cost and eco reasons. However, something I have installed keeps waking it up. I've done a fresh reboot, and then told it to sleep once it's fully up and running and before launching any other apps. It won't stay asleep for as long as a half hour (approximately, I haven't timed it) and it certainly won't stay asleep overnight.
By sleep I mean the monitors go dark and the hard drives spin down and the fan stops and power draw is minimised. By wake up I mean a jet takes off in my Mac Pro tower as all the fans and drives spin up together loudly enough to be heard in another room. The monitors may or may not come on, they seem to sleep all right.
FWIW I'm running Snow Leopard on a first-generation Intel Xeon quad-core Mac Pro with 10 GB of RAM, 4 internal hard drives and two monitors, but I think this is a software thing, not hardware, so that shouldn't matter.
How do I find out what process/app/plugin/background operation/preference pane/whatever is waking up my Mac?